Light-emitting diodes (LED""s) are useful as an illumination source in signs for example, in simple house-number signs. It is desirable that the illumination source be reliable, and have high efficiency, both in terms of light output, and with respect to low power consumption from the power source. Also, low cost is important, in order to have the largest possible market for any product. For the house number sign application, an AC line-operated power source is undesirable, due to both inconvenience and cost.
In response to this need, a high-efficiency, low power consumption, low-cost, LED light source is provided, which is battery-operated, operating from one cell. The inventive voltage-adder, LED driver circuit provides an increased voltage, greater than the battery voltage, for powering the load, in this application, a LED.
The inventive voltage-adder circuit may also be used for providing an increased supply voltage to other circuitry having such a need. Additional applications include use as a battery-voltage booster for watch circuits; use as a back-bias generator for providing faster response time in integrated circuits; and, as a gate-bias-voltage generator, for use with depletion-mode GaAsFETs, such as arc used in the transmitters of cellular telephones. The latter two applications benefit from the provision of a voltage-adder output voltage of opposite polarity to the input supply voltage, rather than providing a voltage-adder output voltage of the same polarity as the input supply voltage.
1. Field of the Invention
She field of the invention is voltage-adder circuits, especially, voltage-adder circuits suitable for operating from a single battery cell, for the purpose of driving a LED, such that the LED provides the appearance of continuous illumination.
Such a voltage-adder circuit is suitable for other applications, as mentioned, and even if not operating from a single cell, but rather, operating from a multiple-cell battery.
Also, the circuit of the present invention may be used to provide an auxiliary output voltage from any power supply, including, for example, AC line-voltage or solar-powered power-supply systems.
2. Description of the Prior Art
Prior-art LED driver circuits generally provide continuous, direct-current drive to the LED, or other load, or provide LED pulsing at a very low rate, say, one blink per second, thereby providing a xe2x80x9cblinking-LEDxe2x80x9d function. DC drive is not suitable for a high-efficiency system, and blinking effect is not suitable for a normal sign illumination source.
Prior art circuits include, capacitive voltage doubler circuits, and flyback switching converter circuits.
Capacitive voltage doubler circuits, whether voltage-inverting, or step-up circuits, are illustrated in FIGS. 2A and 2B, respectively. Both circuits may be arranged to provide, ideally, a maximum output voltage of 2(Vinxe2x88x92VF), where Vin is the battery voltage, and VF is the diode forward voltage drop.
The two forward diode voltage drop is significant with respect to the voltage of one cell. Germanium diodes and Schottky diodes have forward drops of about 0.25 to about 0.3 volts. Twice this is about one-third to one-half of the voltage of a single alkaline cell.
In the prior-art circuit of FIG. 2A, when switch SA is in position 1, capacitor C1 charges through forward-biased diode D1 to (Vinxe2x88x92VF). When switch SA is subsequently connected to the negative input supply terminal 2, the left side of capacitor C1 is connected to zero volts, and since the voltage across a capacitor does not change instantaneously, the voltage at the right-hand side of capacitor C1 is driven to xe2x88x92(Vinxe2x88x92VF). Diode D2 is then forward-biased, charging capacitor C2 to VO=xe2x88x92(Vinxe2x88x922VF), the indicated output voltage across capacitor C2. The circuit is, ideally, a voltage-inverter, in the case of VF=0. Also, the voltage difference between then input supply voltage and Vo is Vinxe2x88x92Vo=Vinxe2x88x92[xe2x88x92(Vinxe2x88x922VF)]=2(Vinxe2x88x92VF), as stated, providing a very poor voltage-doubler circuit, due to the 2VF voltage drop on the two diodes.
In the prior-art circuit of FIG. 2B, when switch SB is connected to position 1, capacitor C1 charges through diode D1 to (Vinxe2x88x92VF), and capacitor C2 charges to (Vinxe2x88x92VF). When switch SB is subsequently connected to position 2, the left-hand side of capacitor C1 is connected to Vin, and diode D1 is reverse-biased. Since the voltage on a capacitor does not change instantaneously, the voltage on the right-hand side of capacitor C1 is (Vin+[Vinxe2x88x92VF)]). Capacitor C2 is charged by capacitor C1 through diode D2 to Vo=2(Vinxe2x88x92VF), as stated, again providing a very poor voltage-doubler circuit.
Flyback switching converter circuits include an inductor, a normally-closed switch, a rectifier diode and an output storage capacitor, and use a controller circuit to provide a regulated DC output voltage. The circuit is expensive for this application.
The present invention provides a circuit for driving a LED from a single-cell battery power source. Since the forward, xe2x80x9cONxe2x80x9d, voltage of the LED is greater than the one-cell battery voltage, the circuit is required to provide an increase of the voltage applied to the LED, of magnitude greater than the single cell voltage.
A novel xe2x80x9cvoltage-adderxe2x80x9d circuit is provided. The inventive voltage-adder circuit provides a greater output voltage than prior art circuits.
A novel, single-cell, battery-operated, voltage-adder circuit is provided for pulsing the LED at a sufficiently rapid rate, that the visual impression of a continuous light source is given.
The goals of the present invention are accomplished by providing a novel voltage-adder circuit, which uses inductors, rather than diodes, in the voltage adding circuitry, thereby providing a greater increase in output voltage than can be achieved with prior art circuits, and with greater efficiency.
One switch is required in the basic circuit. The switch on-state forward voltage drop depends on the switch device chosen, and may typically be about, say, 0.1 volt, small with respect to the voltage of a single cell. Alternative topologies use additional switches, as will be described.
The system incorporates a low-standby drain oscillator, for driving the switches, thus maintaining high system efficiency.
In our application, a rechargeable alkaline cell is used, with solar cell recharging. Oscillator-disabling circuitry is provided, which assures reliable re-starting of the oscillator, upon removal of the oscillator-disable input signal.